The present invention relates to an exposure control method for use with a photographic printer, and more particularly to an exposure control method capable of determining with high precision the setting positions of color correction filters for adjusting color or density.
For a photographic printer of the light adjustment type where three color components of a printing light are adjusted cyan, magenta and yellow color correction filters are disposed between a light source and a negative film. The printing light, of whose three color components the ratio and intensity have been adjusted by these color correction filters, is mixed and applied to an image frame (negative image) set at a printing stage. This image frame then is projected upon a color paper while the shutter is open.
With a conventional open-type photometry and light adjustment method, the three color correction filters are set at predetermined positions and each color of an image frame to be printed is measured with a corresponding light receiving device. The measured density (strictly speaking, a logarithmic value of the light amount) of each color and, if necessary, manually inputted correction data are used to calculate the logarithmic values of exposure light amount for red, green and blue colors. The relationship between the setting positions of the color correction filters, each represented by the number of drive pulses from a pulse motor and the logarithmic values of light amount, is previously stored in a memory as a calibration curve for each color. By referring to the calibration curve, the setting position of each color correction filter can be obtained from the logarithmic value of exposure light amount. The color correction filters then are set at the determined setting positions by driving the pulse motor.
A calibration curve obtained by setting the three color correction filters at the same filter setting position differs from a calibration curve obtained by using the three color correction filters sequentially and separately. The reason for this is that if the three color correction filters are inserted in the printing optical path at the same time, unwanted absorption occurs in those filters. The calibration curve used with the conventional open-type photometry and light adjustment method is formed by using the three color correction filters at the same time. Therefore, although proper color adjustment can be obtained for an image frame having a proper color balance that allows setting the three color correction filters at the same position, proper exposure control cannot be obtained for most of the actual image frames to be printed, since they have unbalanced color.
The above problems can be solved by using feedback control. Specifically, by using as a target value the logarithmic value of calculated exposure light amount for each color, and as an actual measured light value the logarithmic value of light amount measured by inserting each color correction filter into the printing optical path, the position of the color correction filter may be adjusted so as to make the actual measured light value coincident with the target value. However, this feedback control requires a DC servo motor in a drive unit of the color correction filter. A brushless DC servo motor is expensive a brush type DC servo motor is not satisfactorily durable.
In addition to the photometry error caused by unwanted absorption by association of color correction filters, there is another photometry error which depends upon a photometry precision of light receiving devices for measuring an image frame to be printed. For instance, a blue color light receiving device for measuring a blue color has sensitivity not only to essential blue color but also to red color, although the sensitivity to the latter is relatively small. Therefore, it is difficult to measure correctly the blue color component transmitted through an image frame. The sensitivity for a color other than an essential color will be referred to as "sensitivity crosstalk".